1. Field of the Invention
The present invention relates to a switch frequency adjusting system and method, more particularly, to a dynamic switch frequency adjusting system and method for a burst mode of a liquid crystal display.
2. Description of the Related Art
A Cold Cathode Fluorescent Lamp (CCFL) is used in conventional liquid crystal displays as a backlight. The brightness of the CCFL is adjusted by switching the duty cycle of the CCFL driven by an AC source of high voltages. Referring to FIG. 1, in a liquid crystal display, a controller 11 and an inverter 12 are used to light a lamp 13 and to control the brightness of the lamp 13. The controller 11 and the inverter 12 can be combined into a module.
Referring to FIG. 2A and FIG. 2B, there are two modes to light the lamp and control the brightness of the lamp. The first mode is a continuous mode. The waveform of a control signal of the continuous mode is shown in FIG. 2A and the signal has frequencies of 30-70 KHz. The amplitude of the control signal is utilized to tune the brightness of the lamp according to the continuous mode.
The second mode is a burst mode. The waveform of a control signal of the burst mode is shown in FIG. 2B and the frequency of the control signal is also 30-70 KHz. A switch frequency of the control signal is 100-500 Hz, and the switch frequency is controlled to tune the brightness of the lamp by adjusting the duty cycle of the ON/OFF time of the control signal.
Most liquid crystal displays utilize the burst mode. The switch frequency is a certain value, and different manufacturers design the various switch frequency values. The liquid crystal display must receive a scan frequency value from a signal source so as to scan the display. The signal source could be set by a display card of a computer terminal or a software of a computer terminal. Therefore, the scan frequency value is not a fixed value. At some scan frequency values, people can see some water flow on the display. Sometimes the water flow is shifted upwardly, and sometimes the water flow is shifted downwardly. The shifting speed of the water flow is not fixed. According to experiments, the water flow is related to the scan frequency and the switch frequency.
Therefore, it is necessary to provide a dynamic switch frequency adjusting system and method for burst mode so as to solve the above problem.